Use the Advanced Image Search to narrow your search for images online. You can specify the size, file type, usage rights, type of image, country, site, etc.
Use Flickr's Advanced Search to narrow your search for images by content or media type, usage rights, or date.
These websites organize online images for scholarly use with many archival and art images.
More sites that offer images freely available for use.
Many museums are now offering high quality scans of their collections online for use by students and scholars.